Ceramic materials containing certain mixed metal oxide compositions possess both oxygen ion conductivity and electronic conductivity at elevated temperatures. These materials, known in the art as mixed conducting metal oxides, may be used in applications including gas separation membranes and membrane oxidation reactors. These ceramic membranes are made of selected mixed metal oxide compositions and have been described as ion transport membranes (ITM). A characteristic property of these materials is that their oxygen stoichiometry is a thermodynamic function of temperature and oxygen partial pressure wherein the equilibrium oxygen stoichiometry decreases with increasing temperature and with decreasing oxygen partial pressure.
It is known that the dimensions of most materials change with changing temperature due to thermal expansion and contraction. In addition to these thermal dimensional changes, mixed conducting metal oxide materials undergo chemical dimensional changes that are a function of the metal oxide oxygen stoichiometry. At isothermal conditions, an article made of mixed conducting metal oxide material will increase in dimensions with decreasing oxygen stoichiometry. At isothermal conditions, the oxygen stoichiometry decreases with decreasing oxygen partial pressure. Since the equilibrium oxygen stoichiometry increases with decreasing temperature, an article made of mixed conducting metal oxides will contract due to both thermal and chemical dimensional changes as the temperature decreases. Conversely, an article made of mixed conducting metal oxides will expand by both thermal and chemical dimensional changes as the temperature increases at a constant oxygen partial pressure. This is described in an article entitled “Chemical Expansivity of Electrochemical Ceramics” by S. B. Adler in J. Am. Ceram. Soc. 84 (9) 2117-19 (2001).
Dimensional changes therefore result from equilibrium oxygen stoichiometry changes in mixed conducting metal oxide materials. Changing the temperature at a constant oxygen partial pressure or changing the oxygen partial pressure at a constant temperature will change the equilibrium oxygen stoichiometry of the mixed conducting metal oxide material. When a mixed conducting metal oxide is used as an ion transport membrane, for example, an oxygen partial pressure difference across the membrane creates a difference in the equilibrium oxygen stoichiometry at each of the two surfaces of the membrane, which in turn creates the thermodynamic driving force for oxygen ions to diffuse through the membrane.
During startup of a gas separation system using mixed conducting metal oxide membranes, the temperature is increased and the oxygen partial pressure on one or both sides of the membrane may change. The equilibrium oxygen stoichiometry of the membrane material will change in response to the changes in temperature and oxygen partial pressure. Oxygen anions will diffuse into or out of the membrane material and the membrane material will approach its equilibrium oxygen stoichiometry value. As the oxygen stoichiometry and temperature changes, the dimension of the membrane will change. The time required for the membrane to reach chemical equilibrium with the oxygen partial pressures on the surfaces of the membrane will depend on the oxygen anion diffusion rate into or out of the membrane. The time required for equilibration to occur is a function of the material composition, the temperature, and the dimension of the membrane modules.
Different membrane compositions will have different oxygen anion diffusivities, and compositions with higher diffusivities will equilibrate with the gas phase faster, all other factors being equal. For a given membrane composition, the oxygen anion diffusivity increases exponentially with temperature. Therefore, equilibration times decrease with increasing temperature. Finally, the equilibration time increases approximately with the square of the characteristic dimension (e.g., length or thickness) of the parts in the membrane modules. Therefore, thinner parts will equilibrate faster than thicker parts, all other factors being equal. As the thickness of a part increases and as the temperature decreases, it becomes increasingly difficult to keep the interior of the part in equilibrium with the gas phase due to sluggish diffusion of oxygen anions into or out of the part.
It is known that temperature gradients in a mixed conducting metal oxide ceramic part can create differential strains due to differential thermal expansion and contraction. Similarly, oxygen stoichiometry gradients in a ceramic part can create differential strains due to differential chemical expansion and contraction. This gradient in oxygen stoichiometry may be sufficiently large to create a correspondingly large differential chemical expansion, and therefore large mechanical stresses, that lead to failure of the part. Therefore, it is desirable to avoid differential chemical expansion or at least to control the differential chemical expansion to below maximum allowable values.
There is a need in applications of mixed conducting metal oxide ceramics for methods to heat or cool ceramic articles at faster rates without producing unacceptable strains in the articles. One method to achieve this is described in U.S. Pat. No. 7,122,072 B2 that discloses a treatment process in which a mixed conducting metal oxide article is contacted with an oxygen-containing gas while the temperature of the gas is increased or reduced. When the temperature of the oxygen-containing gas is increased or reduced, the oxygen activity in the oxygen-containing gas is increased or reduced, respectively, so that the oxygen activity in the gas is substantially in equilibrium with the stochiometric composition of the mixed conducting metal oxide. In this method, a diluent gas may be used to control the oxygen activity during certain periods in the treatment process.
There also is a continuing need in the art for improved methods to reduce the potential for mechanical damage due to chemical dimensional changes during the heating and cooling of articles and systems fabricated from mixed conducting metal oxide materials, both in the manufacturing of parts for such systems and in the operation of gas separation and membrane reactor systems under transient temperature conditions. In particular, there is a need for improvements to these methods in which a diluent gas is used in various process steps during the heating and cooling of mixed conducting metal oxide materials. This need is addressed by the embodiments of the invention disclosed below and defined by the claims that follow. These embodiments are improvements to the method described in U.S. Pat. No. 7,122,072 B2.